Which of these numbers is prime? ${6,\ 13,\ 51,\ 57,\ 85}$
A prime number has exactly two factors: 1 and itself. The factors of 6 are 1, 2, 3, and 6. The factors of 13 are 1 and 13. The factors of 51 are 1, 3, 17, and 51. The factors of 57 are 1, 3, 19, and 57. The factors of 85 are 1, 5, 17, and 85. Thus, 13 is a prime number.